An ultrasonic motor which is driven by generating travelling vibrational waves by means of electro-mechanical energy conversion element such as piezo-electric element, electrostrictive element, magnetostrictive element has been well known. The ultrasonic motor using a piezo-electric element as a driving source for ultrasonic vibration will be briefly explained. A multiplicity of piezo-electric elements are polarized so that adjacent piezo-electric elements have opposite polarities and the elements are divided into two groups. Stators are secured to an elastic member at an interval of .lambda./4+n.lambda. (wherein .lambda. denotes a wave length of a travelling wave and n denotes an integer). When different alternating voltages Vo sin.theta. and Vo cos.theta. which have the same amplitude and are different by 90.degree. (.pi./2) in phase at a resonant frequency determined by the shape and material of an oscillator comprising the elastic member and piezo-electric elements are applied to each group of electrodes of the respective piezo-electric elements of the stator, an elastic travelling wave is excited on the surface of the elastic member. Each material point on the surface of the elastic member at which the travelling wave is excited performs elliptical vibration in a direction opposite to the travelling direction. Therefore, if a rotor which is in frictional contact with the elastic member is placed around the apex of the peak of the travelling wave, the rotor will be moved in a direction of elliptical vibration. This is a principle of an ultrasonic motor.
Among the ultrasonic motors, the travelling wave type ultrasonic motor and the hybrid oscillator type ultrasonic motor in which vertical vibration and torsion vibration are synthesized require more than one phase alternating voltage. In these ultrasonic motors, theoretically it will suffice to render the phase difference between voltages constant.
However, if there are variations in operating parameters of the finished motors and phase difference between electrical and mechanical vibrations, it will not always suffice to provide a constant phase difference between voltages. Since inappropriate phase difference invites a decrease in energy conversion efficiency and motor output, a circuit may be provided for driving an ultrasonic motor in which phase difference between voltages is adjustable. One example of such driving circuit is disclosed in, for example, Japanese Unexamined Patent Publication No. Sho/62-171475.
The disclosed ultrasonic motor driving circuit effects relative movement between first and second structural members by applying phase different alternating voltages to an electro-mechanical energy conversion element mounted on a first structural member via first and second driving electrodes, respectively to induce travelling vibration waves on the first member. The driving circuit includes a circuit for detecting a deviation between the phase difference between the driving voltages detected at the first and second driving electrodes and a given phase difference and a circuit for regulating the phase differences between alternating voltages applied to the first and second electrodes in response to the output from the deviation detecting circuit.
In order to digitally preset the phase difference between alternating voltages at the regulating circuit in response to the output from the circuit for detecting the deviation of phase difference between alternating voltages, the output frequency of a VCO (voltage controlled oscillator) is divided by 32 by a frequency dividing circuit, so that signals which are shifted in phase by 11.25.degree. (360.degree..div.32) are obtained.
However, phase shift resolution would assume at least only 11.25.degree. if an ultrasonic motor is formed as that disclosed in the above-mentioned Japanese Unexamined Patent Publication No. Sho/62-171475 in which oscillation output of VCO is divided by 32 by a frequency dividing circuit so that signals are phase-shifted by 11.25.degree. (360.degree..div.32). In order to obtain a higher resolution, the oscillation frequency should be increased. In case of a VCO, an increase in oscillation frequency shortens the period of electrical charging and discharging. Therefore, the capacitance used for VCO should be decreased so that the value of the charged and discharged electric current becomes lower.
This provides a lower immunity of the driving circuit to disturbance noise. Since power of several watts to several tens of watts is supplied to the ultrasonic motor, the noises from the supplied power make the oscillation frequency of the VCO unstable. The circuit per se becomes very sensitive to changes in characteristics of the charging and discharging parts, such as capacitors, resistors and transistors. Slight changes in temperature conditions will cause large changes in the oscillation frequency.
If the driving frequency is represented as fo, the oscillation frequency of a VCO is 32fo in case of the aforementioned Unexamined Patent Publication. For example, if fo is 40 kHz, the original oscillation frequency would be 1.28 MHz. The resolution of phase shift is only 11.25.degree. as mentioned above even at such a high frequency oscillation. For a higher resolution, a higher oscillation frequency is necessary. Considering the circuit stability, it is very hard to obtain an oscillation frequency not less than 1 MHz by VCO.
Various functions are often required and restrictions are imposed on the application of an ultrasonic motor. In some type of ultrasonic motor, such as the afore-mentioned travelling wave type ultrasonic motor an optimum driving frequency varies depending upon external conditions such as temperature and external load. The frequency on starting should be preset higher than the driving frequency, otherwise the motor may not be started.
The speed of the ultrasonic motor should often be controlled. In order to control speed of the motor, frequency, phase and power and the like should be appropriately changed.
In case where a plurality of ultrasonic motors such as a motor for winding film and a motor for driving a lens are independently provided in a camera, two ultrasonic motors do not work simultaneously. Accordingly, only one driving circuit suffices if it is capable of appropriately switching the frequency and phase of the driving voltages. Accordingly, an ultrasonic motor driving circuit which is capable of readily switching the frequency, phase and output power is necessary to meet the various requirements by making the best use of the characteristics of the ultrasonic motor.
Addition of the afore-mentioned capabilities to a conventional circuit for driving an ultrasonic motor does not only provide a large and complicated scale of circuit, but also it requires a large space which the circuit occupies and invites high cost. Furthermore it provides more adjusting positions and a more complicated adjusting process, resulting in big problems in mass production. Countermeasure for changes in characteristics of the circuit due to aging of the parts should be taken. This also promotes the necessity of more precise parts and complicated circuits.